Well in general, I don't think that would be too hard to accomplish. A true weaponsmith could certainly device the mechanics to fit into the back of Froggy. There have been far smaller working firearms made in history, so that shouldn't be too big of a problem.
Though still not shooting bullets, I'm tinkering with a motorized version, which will be basically fully functional in the boundaries of a prop including a wrist trigger, but due to lack of time and many projects of higher priority it will be a long time before I get it done.
Maybe a wrist trigger, similar to Spiderman's web canisters, would be practical, yet concealable enough to operate the firing mechanism? Or would it be more like Buzz Lightyear, where you still have to reach over with you other hand, and push a button?
Both. The mechanics I developed for the functional version has two steps: The main trigger is a wrist trigger, though not like spideys moving the hand up (that would be dumb, unless you want to shoot of your own fingers), but down (remember Jango/Boba Fett shooting their wrist gadgets?). But as such wrist triggers have one major handicap, meaning they would constantly be triggered if you do that handmovement, which again isn't practically, I set a second circuit, that basically arms the main trigger, which is the standard pistol trigger on Froggies back. So basically if you want to shoot you pull once the trigger on the Froggies back with the other hand to unlock the weapon after that, you are free to shoot by dropping your hand, activating the motor turning the barrel whenever you want to, until you safeguard the weapon again by pulling Froggies back trigger a second time.
However if the upgrades work out, it will be: a drive will turn the drum and barrels triggered via wrist movement and light fx will be added as well. Of course it still won't fire any bullets (see discussions below).
Materials used: genuine leather, steel, brass, plastic, glass, 2-component sculpting putty and lots of sweat..
You may have trouble covering it with your sleeve lol. It looks like something you'd see in an Assassins' Creed/Sherlock Holmes mashup. Congrats on the DD, by the way. I guess the next step is to make it collapsible.